Category: Genetics

Merle, Merle, On My Dog…Who’s The Cutest of Them All

Meet Our Mascot.

This is Scarlett. Named after The Grateful Dead song, “Scarlett Begonias.” “Scar” for short. “ScarJo” for fun. @scar_struck on instagram. Honestly, it doesn’t matter what you call her: she’ll never be able to hear it.

Scarlett is an Australian Shepherd mix who is blind and deaf, the result of backyard breeding gone wrong.

In the daylight, though, it’s hard to tell. She can play fetch with her tire chew toy, bark at passing firetrucks, and hide from the cats. She’ll follow you around the house when you get up, and don’t even think about leaving food out. But as soon as the lights go out, you can see her have to navigate a whole new world.

Scarlett spent her early months wandering the streets of Mississippi, covered in fleas, ticks, and parasites. She was likely let go after anyone failed to adopt her. Then she was picked up at 4 months old by the amazing people at Wolftrap Animal Rescue in Virginia, beginning her journey to recovery. There it was determined that she is mostly blind – able to see light, shadows, and bright color – and completely deaf.

Despite that, her ears still move, conveying her excitement, curiosity, or dismay, but they can’t send any signal to her brain. And one of her eyes never developed, severely inhibiting her vision, particularly on the left side.

Okay – now for the genetics part.

Basically, every cell you have has two copies of a gene, these copies are known as alleles. These alleles are copies you got from your parents. However, there are a select few cells that have only one copy – for when you’re ready to become a parent. These cells are known as gametes – sperm or egg cells – that have only one allele. This results from a process called meiosis, which splits cells into gametes. Then, when two individuals love each other very much…they combine their gametes via fertilization which creates a zygote with two alleles. This random splitting and combining of alleles is the reason siblings look different and you don’t look like a clone of your parents.

Humans have anywhere between 20,000-25,000 genes according to the Human Genome Project. Dogs have about 19,000.

One of those 19,000 genes is the Merle gene, which affects the production of eumelanin, the dark brown or black pigment protein associated with darker skin and hair and is also responsible for eye development. Its dominant form is denoted by a capital ‘M’ and its recessive counterpart by a lowercase ‘m’. Different types of dominance exist, but thankfully this one is pretty simple: if the dominant merle gene is present, it’ll have an effect on the phenotype, or expression, of the gene more than the recessive allele. Dogs with a marbled coat, referred to as a common Merle, have heterozygous alleles, which just means one dominant and one recessive allele (Mm). Breeding them together is the problem.


Combining two Double Merle (MM) alleles can potentially result in a slew of developmental issues.

A basic punnett square of the merle gene after breeding two heterozygous dogs.
a punnett square

Let’s start here: this is a punnett square, a way of visualizing the genetic probability of a given gene(s). They can get pretty complicated, but this one is simple.

Either side of the square is a parent – mom or dad, and in this case it doesn’t matter which. (Some genes exist exclusively on sex chromosomes, so then it starts to matter which parent, but the merle gene isn’t one of them).

In this case, the parents are each heterozygous for the Merle gene (Mm). These dogs would have marble-colored coats (as shown in the heterozygotes below).

To determine the probability of their offspring’s genotype – or the allele combination they get – use a punnett square. To do so, take one allele from each parent and match it with another in the corresponding square. One ‘M’ from one parent and an ‘m’ from another create heterozygous offspring. In this case, you get two boxes of Mm babies. 2/4 = 1/2 = 50% chance of having a heterozygous offspring. These are healthy, marble coat babies and the reason a lot of backyard breeders breed Merle dogs together in the first place. Do the same for another pair of alleles: one ‘m’ from one parent and another ‘m’ from the other. This combination produces healthy, regular coat babies. 1 square out of 4 = 25% chance of this combo.

I bet you can see where this is going.

This leaves us with our final square. Both ‘M’s from the parents. Here you have the double Merle babies. Also a 25% chance.

So think of it like this:

A punnett square with pictures depicting the different merle phenotypes and the percentage probabilities.
A super visual representation of Merle genetics

If both alleles present are dominant, those big Ms make some big changes. The double dominance is the reason Scarlett’s coat is mostly white with marble coloring, why her left eye is underdeveloped, and why her ears can’t pick up noise. Double Merles can’t generate enough eumelanin, meaning their hair and skin lack pigment and the eyes don’t develop properly. This also underlies albinism.

So, fun fact: pigment follicles in your ear are involved with translating the vibrations of sound into a neural signal. Thus, missing these pigmented cells – known as cilia – can mean deafness. Double Merles often suffer from this.

Double Merle dogs often have blue eyes because of the lack of pigment in their eyes. This can also cause eye developmental issues including corectopia, where the pupil is subluxated or out of place, starburst pupils where the pupil is spread out with irregular edges, and micropthalmia a condition resulting in small eyes.

In Scarlett’s case, her left eye is both subluxated and starburst, which causes her blindness.

Scarlett’s pupils are both subluxated and starburst.

Double Merle dogs are a prime example why understanding genetics is necessary when breeding dogs – especially given how many animals are killed every year when they can’t get placed in homes.

While genetics may seem overwhelming, the discipline is vital in determining the etiology of genetic disorders, the effects of our environment on our genes, and it’s the whole principle behind ancestry tests.

I met Scarlett in late April 2019 when my neighbor was fostering her. And it was love at first sight meet. My boyfriend and I adopted her a few weeks later. Even people who don’t normally love dogs, can’t help but be taken by her sprightly personality.

At a year old, Scarlett is a well-behaved, loving little tank with some of the strongest back legs I’ve seen on a dog. She’s a thick 40lbs and has a knack for chewing (everything). She’s done growing, so her ears are perpetually large and animated and her claws are like talons. She rarely barks, except when she ‘hears’ a high pitch or when she’s dreaming. She loves to sleep under a blanket, has to be touching someone at all times, and loves her stegosaurus chew toy. And she hates the rain.

The most common question we get asked is how we train her. Simply, using tactile stimulation with butt scratches or a spray bottle of water, depending on what she’s doing. Oh and lots of treats.

She really is a normal dog, who just so happens to not be able to hear or see very well. She’s clever, entertaining, and unconditionally loving. Proof it’s nurture and nature.

Remember to adopt, don’t shop! Contact your local animal rescue if you’re looking for a new furry friend.